It is known in the art that the power required to compress a gas can be reduced by compressing the gas in stages in order to allow for cooling of the gas between stages. Eventually a balance is reached where the power savings are offset by the capital cost of dividing the compression step into more and more stages, but depending on the compression duty at issue and the relative costs of power vs. capital, the optimum number of stages will often be several. This is particularly true in the case of compressing an air stream that is fed to a typically sized cryogenic air separation unit (“ASU”) wherein the air stream is separated into one or more product streams typically including at least a nitrogen product and an oxygen product, often an argon product, and less often krypton and xenon products.
It is also known in the art that the power savings are proportional to the inter-stage cooling temperature. In particular, cooling to a sub-ambient temperature between stages with a refrigerant such as liquefied natural gas (“LNG”) will yield greater power savings than cooling to ambient temperature by using ordinary cooling water as the refrigerant. Once again, eventually a balance is reached where the power savings are offset by the capital cost of the additional refrigeration required to cool the inter-stage gas to a colder and colder temperature. Typically, this balance does not justify the use of anything colder than ambient temperature cooling water. A notable exception however is in the context of an ASU located near an LNG terminal. In such a case, the cost of the LNG is often low enough to not only justify the use of LNG, but to also justify as much LNG as is required to cool the inter-stage air stream to a temperature just above the freezing point of the contaminants contained in the air stream, particularly water and carbon dioxide.
As used herein (and as generally referred to in the industry), “cold compressing” shall mean compression of a gas that is at a sub-ambient temperature at the inlet of a compressor stage. (Contrast this term with “warm compressing” which is the industry term for compression of a gas that is at approximately ambient temperature or above ambient temperature at the inlet of a compressor stage.) Also as used herein, “natural gas refrigeration” shall mean either (i) refrigeration in the form of LNG or (ii) refrigeration in the form of a cold (i.e. a temperature below ambient, especially well below ambient) natural gas, especially the cold natural gas that results from vaporized, but only partially warmed, LNG. For example, the cold natural gas is at a temperature of −20° C. to −120° C., preferably −40° C. to −100° C.
The present invention relates to a system that uses natural gas refrigeration to cold compress an air stream, especially an air stream which is subsequently fed to an ASU. The art teaches such a system. See for example FIG. 1 of Japanese Patent Application 53-124188 by Ishizu (hereafter “Ishizu”) and U.S. Pat. No. 3,886,758 by Perrotin et al. (hereafter “Perrotin”).
Ishizu refers to a prior art cryogenic air separation process (see FIG. 1) in which LNG is used to provide inter-stage cooling during compression of wet feed air for an ASU incorporating a distillation column system and teaches that the problem of moisture and carbon dioxide freezing during the inter-stage cooling in that process can be obviated by using the LNG to remove heat generated by compression of dry feed air that has been cooled to about −150° C. instead of for the inter-stage cooling (see FIG. 2). The LNG cools the compressed air back to about −150° C. and the resultant cooled compressed air is subsequently cooled to about −170° C. before feeding to the distillation column system.
Perrotin discloses a cryogenic air separation process in which LNG is used to provide condensation duty to a compressed nitrogen product stream from a distillation column system to provide a reflux stream to the distillation column system. Optionally, LNG also is used to provide inter-stage cooling of dried air during feed air compression.
A common concern in Ishizu and Perrotin is the exposure to a scenario where a defect in the heat exchanger used to facilitate the heat exchange between the LNG and inter-stage air stream results in natural gas leaking into the air stream. In particular, such a leak would permit natural gas to enter the distillation column along with the air stream where the natural gas will tend to collect with the oxygen produced in the distillation column and thus create potentially explosive mixtures of oxygen and natural gas. It is an object of the present invention to address this concern.
The art also teaches the use of LNG to cool the air stream after its last stage of compression (hereafter, the “finally compressed air stream”). See for example U.S. Pat. No. 4,192,662 by Ogata et al. (hereafter “Ogata”) and US Patent Application 2005/0126220 by Ward (hereafter “Ward”).
Ogata discloses a cryogenic air separation process in which LNG is used to cool a circulating nitrogen product stream whereby the stream can be compressed at low temperature and expanded to vaporize oxygen in a rectifying column. In the exemplified process, LNG also is used to provide refrigeration duty to a closed chlorofluorocarbon cycle that in turn provides refrigeration duty to the finally compressed air stream.
Ward discloses a method of adjusting the gross heating value of LNG by adding a condensable gas whereby at least a portion of that gas is condensed by the LNG to provide a blended condensate, which is subsequently vaporized by heat exchange with a heat transfer medium. The heat transfer medium can be used, for example, as a coolant to condition an air feed or other process stream associated with a cryogenic air separation or to cool the condensing gas. In the exemplified process, water and/or ethylene glycol is used as the heat transfer medium and portions thereof are used to cool both finally compressed air stream and a compressed nitrogen product stream.
One notable feature in both Ogata and Ward is the use of an intermediate cooling medium (ICM) to transfer the refrigeration from the LNG to the finally compressed air stream. In particular, the ICM is cooled by indirect heat exchange against the LNG in a first heat exchanger and the resulting cooled ICM is used to cool the finally compressed air stream by indirect heat exchange in a second heat exchanger. In this fashion, Ogata and Ward are protected from a scenario where a leak in the heat exchanger used to cool the finally compressed air stream results in natural gas entering the distillation column. It needs to be clearly noted however that Ogata and Ward do not teach to use the cooled ICM to advantageously cool the air stream between its stages of cold compression.
Finally, the art also teaches the use of cold natural gas for inter-stage cooling during cold compression of nitrogen gas. For example U.S. Pat. No. 5,141,543 by Agrawal et al. (hereafter “Agrawal”) refers to a prior art process for liquefaction of nitrogen product streams from a cryogenic air separation in which the nitrogen product streams are cold compressed using a closed chlorofluorocarbon cycle to provide inter-stage cooling and LNG provides refrigeration duty to the chlorofluorocarbon cycle. Additionally, the LNG provides refrigeration for cooling of the finally compressed nitrogen. It needs to be clearly noted that Agrawal does not teach to use the cooled chlorofluorocarbon ICM of the prior art to advantageously provide inter-stage cooling for cold compression of the air stream fed to the ASU.